asdccasca

hi. I have a 1993 yota p/u with 3vze in it and did a head gasket rebuild- new water pump belts- ideler pulley-heads resurfaced- falpro hg kit-new radiator-new plugs. and it started up and runs good but when i went to go do a oil change i poured some stuff to take the old gunked up oil in the crank case out and put a new filter and oil in it. but when i poured the oil treatment into the crankcase it started leaking a little coolant at the back of the engine. so i finished the job up and drove it and its stopped leaking. any ideas what it could of possibly be.
thank you. any feed back is apprieacated!!!!!!!